In the steps for fabricating an integrated circuit, layers of semiconductor materials or metallic layers have to be lithographically patterned. These layers can have variable topographies, either plane or in relief. Conventionally, the lithography is carried out by depositing a photosensitive reason on this layer to be lithographically patterned, only certain desired areas of which are sensitized, these being subsequently developed in a developer appropriate to the chemical nature of the resin, thus revealing specific areas of the layer to be lithographically patterned.
This technique is used, for example, to define the level of the gate of an MOS transistor in a layer of polycrystalline silicon. The dimensions of the gates of a transistor are so small that the slightest spurious exposure is deleterious and unacceptable in transistors. This spurious exposure is, in particular, generated by reflection of incident light from the layer to be lithographically patterned subjacent to the resin, increasing the interference effects in the resin and the scattering of the light on the topological variations. Such effects are becoming less and less tolerable because of the dimensions characteristic of submicron technologies.
One approach for reducing the influence of reflected light in photolithography techniques consists in depositing an antireflection layer called an ARC (antireflective coating), which may either be inorganic or organic, directly on the reflective surface. The photosensitive resin in then deposited directly on the ARC layer.
However, the use of these ARCs requires a separate deposition and annealing steps appropriate to these materials. Apart from the expense of the additional equipment necessary for implementing these deposition techniques, it is necessary to add the precautions to be taken for controlling all the parameters of the process of depositing these layers (annealing times, defect minimization, etc.).
In addition to these problems of implementation of the process, ARCs do not have the properties anticipated.
Although they have good antireflective properties, organic ARCs have problems of variation in the thickness of the layers, due in particular to the topography of the surface on which they are deposited. These problems are in fact directly associated with the effect of deposition planarization. Since the thickness of the ARC is not the same over the entire surface, there is a risk of side overetching in the areas where the ARC is thinner during etching of the ARC by a specific plasma.
With regard to inorganic ARCs, these require adjustments and steps additional to the photolithography process, in particular adjustment of their refractive index n and their extinction coefficient k depending on the thicknesses deposited, a specific etching step after exposure and development of the photosensitive resin, and their removal after photoetching is an additional step. On the other hand, they have the advantage over organic ARCs of giving conformal coatings.
Another approach for reducing the reflection of incident light on a reflective layer in a photolithographic etching step is described in U.S. Pat. No. 5,139,974. This approach consists in increasing the roughness of a reflective metal surface so as to increase the absorption of incident light. However, the roughness of the surface is increased by a process which does not guarantee uniformity of the physical state of the surface, a phenomenon which is aggravated by the fact that the layer to be treated includes features and therefore has a surface in relief. This technique modifies the optical properties of the material only in a random manner which is difficult to reproduce.